Barium titanate series ceramics having a positive temperature coefficient of electric resistance

ABSTRACT

Barium titanate series semiconductive ceramics having a positive temperature coefficient of electric resistance are disclosed. The ceramics consist mainly of a barium titanate series compound and contain specifically limited small amounts of titanium dioxide, silicon oxide, aluminum oxide, at least one element selected from the group consisting of rare earth elements, yttrium, bismuth, antimony, niobium, tantalum and tungsten, at least one element selected from the group consisting of manganese, copper, iron and chromium, and zinc oxide. The ceramics are low in the variance of electric resistance values and have a low water absorption and hence are low in the change of electric properties due to the lapse of time.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to barium titanate series semiconductiveceramics having a positive temperature coefficient of electricresistance (hereinafter, ceramics having a positive temperaturecoefficient of electric resistance are referred to as PTC ceramics),which can be produced from inexpensive industrial raw materials, are lowin the variance of electric resistance, have a low water absorption andhence are low in the change of electric properties due to the lapse oftime.

(2) Description of the Prior Art

It has hitherto been known that barium titanate series semiconductivePTC ceramics can be obtained by sintering a barium titanate seriescompound together with a very small amount of an oxide of rare earthelements, such as lanthanum (La), cerium (Ce), samarium (Sm) and thelike, an oxide of trivalent or pentavalent materials, such as bismuth(Bi), antimony (Sb), niobium (Nb) and the like, or an oxide, such as Ag₂O, SiO₂, B₂ O₃, Al₂ O₃ or the like.

However, the use of inexpensive industrial raw materials containing animpurity element, such as Li, Na, K or the like, which hinders theconversion of barium titanate series ceramic composition intosemiconductor, in the production of these barium titanate seriessemiconductive ceramics has the following drawbacks. That is, bariumtitanate series semiconductive ceramics cannot be produced stably due tothe influence of the impurity, the production of barium titanate seriessemiconductive ceramics having a uniform electric resisrance isdifficult due to the difference of properties of barium carbonate andtitanium dioxide used as main raw materials in different lots, and theresulting semiconductive ceramics have a high water absorption and henceare noticeable in the change of properties due to the lapse of time.Particularly, the use of inexpensive industrial raw materials containingthe above described impurity elements is not suitable for the productionof a large amount of PTC ceramics having a large heat release value,which are known as a honeycomb structural heater (U.S. Pat. No.3,927,300).

SUMMARY OF THE INVENTION

The present invention aims to overcome the above described drawbacks andto provide PTC ceramics, which consist mainly of a barium titanateseries compound and have a predetermined withstand voltage necessary forthe use as a heater.

The feature of the present invention is the provision of barium titanateseries ceramics having a positive temperature coefficient of electricresistance, consisting of

(a) 0.003-0.05 mole, preferably 0.005-0.03 mole, more preferably0.015-0.025 mole, of titanium dioxide (TiO₂),

(b) 0.005-0.05 mole, preferably 0.01-0.03 mole, more preferably0.015-0.025 mole, silicon oxide (SiO₂),

(c) 0.003-0.07 mole, preferably 0.01-0.05 mole, more preferably0.02-0.03 mole, of aluminum oxide (Al₂ O₃),

(d) 0.0014-0.008 mole, preferably 0.004-0.0065 mole, more preferably0.005-0.006 mole, of at least one element selected from the groupconsisting of rare earth elements, yttrium (Y), bismuth (Bi), antimony(Sb), niobium (Nb), tantalum (Ta) and tungsten (W),

(e) 0.00025-0.002 mole, preferably 0.0003-0.0015 mole, more preferably0.0005-0.0008 mole, of at least one element selected from the groupconsisting of manganese (Mn), copper (Cu), iron (Fe) and chromium (Cr),

(f) 0.0001-0.003 mole, preferably 0.0003-0.002 mole, more preferably0.0005-0.001 mole, of zinc oxide, and

(g) [1-((a)+(b)+(c)+(d)+(e)+(f))] mole of a barium titanate seriescompound,

provided that the number of moles given by (d)-((e)+(f)) is0.001-0.00765 mole, preferably 0.003-0.005 mole, more preferably0.0035-0.0047 mole.

The barium titanate series compound to be used in the present inventionincludes barium titanate itself and barium titanate, whose barium ortitanium is partly substituted by elements, such as strontium, lead,zirconium, tin and the like, which shift the Curie temperature to highertemperature side or to lower temperature side. For example, the bariumtitanate series compounds are represented by the following chemicalformulae, BaTiO₃, Ba_(1-x) Sr_(x) TiO₃, Ba_(1-y) Pb_(y) TiO₃, BaTi_(1-z)Zr_(z) O₃, BaTi_(1-w) Sn_(w) O₃, Ba_(1-x-y) Sr_(x) Pb_(y) TiO₃ and thelike, wherein x, y, z and w represent x=40 atom%, y≦70 atom%, z≦20 atom%and w≦25 atom%.

In the barium titanate series PTC ceramics of the present invention,TiO₂ serves to lower the firing temperature of the ceramic compositionand to convert stably the ceramic composition into a semiconductor. Whenthe amount of TiO₂ is less than 0.003 mole, the effect of TiO₂ does notappear, while when the amount is more than 0.05 mole, the resultingceramics become insulating materials. Therefore, the amount of TiO₂should be limited to 0.003-0.05 mole.

Al₂ O₃ is used in an amount of 0.003-0.07 mole. Al₂ O₃ serves to lowerthe firing temperature, to shift the maximum resistance-givingtemperature in the resistance-temperature property curve of the ceramicsto higher temperature side, and further to suppress the influence ofalkali metals, such as K, Na, Li and the like, which are contained inthe raw materials and hinder the conversion of the ceramic compositioninto a semiconductor. When the amount of Al₂ O₃ is less than 0.003 mole,the effect of Al₂ O₃ does not appear, while when the amount exceeds 0.07mole, the ceramic composition is difficult to be stably converted into asemiconductor, or the resulting semiconductive ceramics are low in therate of change of electric resistance corresponding to temperaturechange.

SiO₂ is used in an amount of 0.005-0.05 mole. SiO₂ serves to produceceramics having a high resistance against high voltage and to increasethe rate of change of electric resistance value of the resultingceramics corresponding to temperature change. When the amount of SiO₂ isless than 0.005 mole, the effect of SiO₂ does not appear, while when theamount is more than 0.05 mole, the ceramic composition is difficult tobe stably converted into a semiconductor, or the resultingsemiconductive ceramics have a water absorption of higher than 1%, andfurther the ceramic composition has a low melting point and hence thecomposition is easily fused at the firing.

At least one of rare earth elements and elements of Y, Bi, Sb, Nb, Taand W, which promote the conversion of the ceramic composition into asemiconductor by the control of the valence, is used in an amount of0.0014-0.008 mole in total. These elements serve to produce stablesemiconductive ceramics. When the amount of these elements is less than0.0014 mole, the effect of these elements does not appear, while whenthe amount exceeds 0.008 mole, the resulting ceramics have a very highspecific resistance at room temperature or the ceramic composition isdifficult to be converted into a stable semiconductor.

At least one of elements of Mn, Cu, Fe and Cr is used in an amount of0.00025-0.002 mole in total. These elements serve to produce ceramicshaving a low water absorption and a high withstand voltage and furtherto make the resulting PTC ceramics more homogeneous. When the amount ofthese elements is less than 0.00025 mole, the effect of these elementsdoes not appear, while when the amount exceeds 0.002 mole, ceramicshaving stable properties cannot be obtained or the resulting ceramicsconvert into insulating materials.

The rare earth elements and the elements of Mn, Cu and the like can beused in the form of an oxide or in the form of a salt thereof or anaqueous solution of the salt, which can be converted into an oxide bycalcination or firing. For example, manganese can be used in the form ofan aqueous solution of manganese sulfate.

Zinc oxide is used in an amount of 0.0001-0.003 mole. Zinc oxide servesto decrease the water absorption and the variance of resistance valuesin the resulting ceramics. When the amount of zinc oxide is less than0.0001 mole, the above described effect of zinc oxide does not appear.While, when the amount exceeds 0.003 mole, the resulting ceramics arelow in the water absorption, but are high in the specific resistancevalue at room temperature and are low in the withstand voltage.

In the present invention, the element (d), which promotes the conversionof ceramic composition into semiconductor by the control of the valence,the element (e), which hinders the conversion of ceramic compositioninto semiconductor but serves to make the resulting ceramics morehomogeneous, and zinc oxide (f) must be used in such relative amountsthat the number of moles given by (d)-((e)+(f)) is within the range of0.001-0.00765 mole. When the number of moles given by (d)-((e)+(f)) isless than 0.001 mole or exceeds 0.00765 mole, the specific resistance ofthe resulting PTC ceramics is higher than 10⁵ -10⁶ Ω·cm and the ceramicscannot be practically used as a semiconductor. A particularly preferablerange of number of moles given by (d)-((e)+(f)), which gives asemiconductor having a specific resistance of lower than 10³ Ω·cm, is0.003-0.005 mole.

The following examples are given for the purpose of illustration of thisinvention and are not intended as limitations thereof.

BaCO₃, TiO₂, PbO, SiO₂, Al₂ O₃, Sb₂ O₃, an aqueous solution of manganesesulfate, and ZnO, were weighed so that the resulting mixtures had acomposition shown in the following Table 1, and each of the mixtures wasmixed and dried in a conventional manner, and then calcined at 1,050° C.for 3 hours and further fired at 1,300° C. for 2 hours to obtain bariumtitanate series semiconductive PTC ceramics having a diameter of 18 mmand a thickness of 2.5 mm. Among the resulting PTC ceramics, only thePTC ceramics having a composition defined in the present inventionsatisfy all the following properties, that is, not higher than 1% ofwater absorption, not higher than 10⁵ Ω·cm of specific resistance afterprovided with ohmic electrodes, not more than 35% of variance ofspecific resistance, not lower than 200 V of withstand voltage againstbreakdown, not higher than 40% of rate of change of specific resistanceafter repeating 8,000 times of cycles, each cycle consisting of 100 Vapplication for 1 minute and non-application of voltage for 4 minutes,as shown in Table 1.

When rare earth elements, such as La, Y and the like, were used in placeof Sb as a component for promoting the conversion of ceramic compositioninto semiconductor by the control of the valence, the same result wasobtained as well. When Cu, Fe or Cr was used in place of Mn in order tomake the resulting PTC ceramics more homogeneous, a good result wasobtained as illustrated in Examples 19-21 as well. Further, when theamount of element, which shifts the Curie temperature, was changed, theresulting ceramics as well were satisfactory in the properties aimed inthe present invention as illustrated in Examples 22-26 and Comparativeexamples 13 and 14.

                                      TABLE 1-1                                   __________________________________________________________________________                                                       Vari-   Rate of                                                        Wa-    ance    change                                                         ter                                                                              Spe-                                                                              of spe- of                                                             ab-                                                                              cific                                                                             cific                                                                             With-                                                                             specific                                                       sorp-                                                                            resis-                                                                            resis-                                                                            stand                                                                             resis-                    Barium titanate              ZnO     tion                                                                             tance                                                                             tance                                                                             voltage                                                                           tance                     series compound                                                                        TiO.sub.2                                                                        SiO.sub.2                                                                        Al.sub.2 O.sub.3                                                                  (d)  (e)  (f)                                                                              d-(e+f)                                                                            (%)                                                                              (Ω·cm)                                                             (%) (V) (%)                __________________________________________________________________________    Comparative                                                                   example 1                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0  0.02*                                                                            0.022                                                                             Sb0.005                                                                            Mn0.0005                                                                           0.001                                                                            0.0035                                                                             1.1                                                                               310                                                                              46  200 73                 Example 1                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.003                                                                            0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.4                                                                               82 31  270 13                 Example 2                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.3                                                                               120                                                                              24  350 10                 Example 3                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.05                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.9                                                                               530                                                                              22  230 17                 Comparative                                                                   example 2                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.07                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             -- 10.sup.6 <                                                                        --  --  --                 Comparative                                                                   example 3                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0  0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.1                                                                               15 18  120  7                 Example 4                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.005                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.1                                                                               24 18  240  9                 Example 5                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.018                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.2                                                                               105                                                                              23  330 12                 Example 6                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.05                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             1.0                                                                               930                                                                              33  500 40                 Comparative                                                                   example 4                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.08                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             2.3                                                                              10.sup.5 <                                                                        --  --  --                 Comparative                                                                   example 5                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0   0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             1.8                                                                              10.sup.6 <                                                                        --  --  --                 Example 7                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.003                                                                             0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             1.0                                                                              3400                                                                              31  540 27                 Example 8                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.02                                                                              0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.3                                                                               130                                                                              23  310 13                 Example 9                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.07                                                                              0.005                                                                              0.0005                                                                             0.001                                                                            0.0035                                                                             0.4                                                                               290                                                                              25  320  9                 __________________________________________________________________________

                                      TABLE 1-2                                   __________________________________________________________________________                                                        Vari-                                                                         ance                                                                          of     Rate of                                                         Wa-    spe-   change                                                          ter                                                                              Spe-                                                                              cific  of                                                              ab-                                                                              cific                                                                             re-                                                                              With-                                                                             specific                                                        sorp-                                                                            resist-                                                                           sist-                                                                            stand                                                                             resist-                   Barium titanate              ZnO      tion                                                                             ance                                                                              ance                                                                             voltage                                                                           ance                      series compound                                                                        TiO.sub.2                                                                        SiO.sub.2                                                                        Al.sub.2 O.sub.3                                                                  (d)  (e)  (f) d-(e+f)                                                                            (%)                                                                              (Ω.cm)                                                                      (%)                                                                              (V) (%)                __________________________________________________________________________    Comparative                                                                   example 6                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.09                                                                              Sb0.005                                                                            Mn0.0005                                                                           0.001                                                                             0.0035                                                                             0.2                                                                              10.sup.5 <                                                                        -- --  --                 Comparative                                                                   example 7                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0    0.0005                                                                             0.001                                                                             --   -- 10.sup.6 <                                                                        -- --  --                 Example 10                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.003                                                                              0.0005                                                                             0.001                                                                             0.0015                                                                             0.1                                                                               570                                                                              34 380 17                 Example 11                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.006                                                                              0.0005                                                                             0.001                                                                             0.0045                                                                             0.4                                                                               110                                                                              21 330 12                 Example 12                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.008                                                                              0.0005                                                                             0.001                                                                             0.0065                                                                             0.9                                                                              2700                                                                              35 400 26                 Comparative                                                                   example 8                                                                            Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.012                                                                              0.0005                                                                             0.001                                                                             0.0105                                                                             -- 10.sup.6 <                                                                        -- --  --                 Comparative                                                                   example 9**                                                                          Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0    0.001                                                                             0.004                                                                              0.7                                                                              1800                                                                              71 300 25                 Example 13                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0003                                                                             0.001                                                                             0.0037                                                                             0.5                                                                               84 30 310 13                 Example 14                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.001                                                                              0.001                                                                             0.003                                                                              0.3                                                                               210                                                                              24 470 12                 Example 15                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.002                                                                              0.001                                                                             0.002                                                                              0.2                                                                              1600                                                                              19 520 12                 Comparative                                                                   example 10                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0035                                                                             0.001                                                                             0.0005                                                                             0.2                                                                              10.sup.5 <                                                                        -- --  --                 Comparative                                                                   example 11***                                                                        Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0   0.0045                                                                             1.4                                                                               120                                                                              83 260 10                 Example 16                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.0001                                                                            0.0044                                                                             0.8                                                                               140                                                                              33 290 12                 __________________________________________________________________________

                                      TABLE 1-3                                   __________________________________________________________________________                                                        Vari-                                                                         ance                                                                          of     Rate of                                                         Wa-    spe-   change                                                          ter                                                                              spe-                                                                              cific  of                                                              ab-                                                                              cific                                                                             re-                                                                              With-                                                                             specific                                                        sorp-                                                                            resist-                                                                           sist-                                                                            stand                                                                             resist-                   Barium titanate              ZnO      tion                                                                             ance                                                                              ance                                                                             voltage                                                                           ance                      series compound                                                                        TiO.sub.2                                                                        SiO.sub.2                                                                        Al.sub.2 O.sub.3                                                                  (d)  (e)  (f) d-(e+f)                                                                            (%)                                                                              (Ω.cm)                                                                      (%)                                                                              (V) (%)                __________________________________________________________________________    Example 17                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             Sb0.005                                                                            Mn0.0005                                                                           0.0015                                                                            0.003                                                                              0.3                                                                              150 20 300 11                 Example 18                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.003                                                                             0.0015                                                                             0.2                                                                              380 16 270 11                 Comparative                                                                   example 12                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              0.0005                                                                             0.005                                                                             --   0.1                                                                              2700                                                                              19 180 16                 Example 19                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              Cu0.0005                                                                           0.001                                                                             0.0035                                                                             0.3                                                                              110 21 310 11                 Example 20                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              Fe0.001                                                                            0.001                                                                             0.003                                                                              0.3                                                                              130 24 290 11                 Example 21                                                                           Ba.sub.0.84 Pb.sub.0.16 TiO.sub.3                                                      0.02                                                                             0.02                                                                             0.022                                                                             0.005                                                                              Cr0.001                                                                            0.001                                                                             0.003                                                                              0.3                                                                              110 23 290 10                 Comparative                                                                   example 13***                                                                        Ba.sub.0.95 Pb.sub.0.05 TiO.sub.3                                                      0.02                                                                             0.027                                                                            0.022                                                                             0.005                                                                              Mn0.0005                                                                           0   0.0045                                                                             1.4                                                                              820 55 440 47                 Example 22                                                                           Ba.sub.0.95 Pb.sub.0.05 TiO.sub.3                                                      0.02                                                                             0.027                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.0001                                                                            0.0044                                                                             0.9                                                                              110 35 330 38                 Example 23                                                                           Ba.sub.0.95 Pb.sub.0.05 TiO.sub.3                                                      0.02                                                                             0.027                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.0005                                                                            0.004                                                                              0.8                                                                              150 27 370 24                 Example 24                                                                           Ba.sub.0.95 Pb.sub.0.05 TiO.sub.3                                                      0.02                                                                             0.027                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.001                                                                             0.0035                                                                             0.5                                                                              140 26 380 17                 Example 25                                                                           Ba.sub.0.95 Pb.sub.0.05 TiO.sub.3                                                      0.02                                                                             0.027                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.002                                                                             0.0025                                                                             0.3                                                                              510 22 320 16                 Example 26                                                                           Ba.sub.0.95 Pb.sub.0.05 TiO.sub.3                                                      0.02                                                                             0.027                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.003                                                                             0.0015                                                                             0.1                                                                              1600                                                                              22 230 16                 Comparative                                                                   example 14                                                                           Ba.sub.0.95 Pb.sub.0.05 TiO.sub.3                                                      0.02                                                                             0.027                                                                            0.022                                                                             0.005                                                                              0.0005                                                                             0.005                                                                             --   0.1                                                                              10.sup.6 <                                                                        -- --  --                 __________________________________________________________________________     Note:                                                                         *All the numerical values mean number of moles. The amount of barium          titanate series compound is the remainder when the total amount of the        ceramics is calculated as 1 mole.                                             **A thin insulating layer was formed on the surface of the resulting          ceramics, and the electric properties were measured after removing the        layer.                                                                        ***The color of the resulting ceramics was not uniform.   As described        above, in the present invention, among the components for forming barium     titanate series semiconductive PTC ceramics, that is, among (a) titanium     dioxide, (b) silicon oxide, (c) aluminum oxide, (d) a component for     promoting the conversion of ceramic composition into semiconductor, (e) a     component for making the resulting ceramics homogeneous, (f) zinc oxide     and (g) a barium titanate series compound, the amount of zinc oxide is     particularly limited, whereby barium titanate series semiconductive PTC     ceramics having a low water absorption, which are low in the variance of     electric resistance value and in the change of electric properties due to     the lapse of time, can be obtained. Moreover, since the relative amounts     of the components (d), (e) and (f) are limited so that the number of moles     given by (d)-((e)+(f)) lies within the specifically limited range, barium     titanate series semiconductive PTC ceramics having a low specific     resistance and a low variance of the specific resistance can be stably     produced from inexpensive industrial raw materials. The ceramics can be     used in various heaters, and are very useful for industrial purpose.

What is claimed is:
 1. Barium titanate series ceramics having a positivetemperature coefficient of electric resistance, consisting of(a)0.003-0.05 mole of titanium dioxide, (b) 0.005-0.05 mole of siliconoxide, (c) 0.003-0.07 mole of aluminum oxide, (d) 0.0014-0.008 mole ofat least one element selected from the group consisting of rare earthelements, yttrium, bismuth, antimony, niobium, tantalum and tungsten,(e) 0.00025-0.002 mole of at least one element selected from the groupconsisting of manganese, copper, iron and chromium, (f) 0.0001-0.003mole of zinc oxide, and (g) [1-((a)+(b)+(c)+(d)+(e)+(f))] mole of abarium titanate series compound,provided that the number of moles givenby (d)-((e)+(f)) is 0.001-0.00765 mole.
 2. Barium titanate seriesceramics having a positive temperature coefficient of electricresistance, consisting of(a) 0.005-0.03 mole of titanium dioxide, (b)0.01-0.03 mole of silicon oxide, (c) 0.01-0.05 mole of aluminum oxide,(d) 0.004-0.0065 mole of at least one element selected from the groupconsisting of rare earth elements, yttrium, bismuth, antimony, niobium,tantalum and tungsten, (e) 0.0003-0.0015 mole of at least one elementselected from the group consisting of manganese, copper, iron andchromium, (f) 0.0003-0.002 mole of zinc oxide, and (g)[1-((a)+(b)+(c)+(d)+(e)+(f))] mole of a barium titanate seriescompound,provided that the number of moles given by (d)-((e)+(f)) is0.003-0.005 mole.
 3. Barium titanate series ceramics having a positivetemperature coefficient of electric resistance, consisting of(a)0.015-0.025 mole of titanium dioxide, (b) 0.015-0.025 mole of siliconoxide, (c) 0.02-0.03 mole of aluminum oxide, (d) 0.005-0.006 mole of atleast one element selected from the group consisting of rare earthelements, yttrium, bismuth, antimony, niobium, tantalum and tungsten,(e) 0.0005-0.0008 mole of at least one element selected from the groupconsisting of manganese, copper, iron and chromium, (f) 0.0005-0.001mole of zinc oxide, and (g) [1-((a)+(b)+(c)+(d)+(e)+(f))] mole of abarium titanate series compound,provided that the number of moles givenby (d)-((e)+(f)) is 0.0035-0.0047 mole.